Proportional control valves can be actuated by proportional solenoids to regulate back pressure from remote devices. For example, my copending U.S. patent application Ser. No. 505,889, filed Apr. 6, 1990, now U.S. Pat. No. 5,123,718, discloses a solenoid actuated valve for controlling a flow of fluid under pressure to a brake cylinder. The flow of fluid is maintained until back pressure from the brake cylinder reaches a desired proportional relationship with an increased force exerted by the solenoid. Back pressure is reduced by exhausting fluid flowing from the brake cylinder until the back pressure returns to the desired proportional relationship with a decreased solenoid force.
However, fluid flows between the proportional valves and remote devices are accompanied by friction, which causes a pressure drop between the source and the destination of the fluid flow. Accordingly, pressure changes in remote devices such as brake cylinders or other closed volume loads tend to lag behind the changes in back pressure regulated by proportional control valves. This can cause instability in the operation of the valves.
While charging, for example, proportional control valves can momentarily close off the supply of pressurized fluid to the remote devices before the remote devices have reached the desired pressure level. Upon closing the supply, static pressure within the valves approaches the prior average dynamic pressure of the valves and remote devices, which is below the desired pressure level and can cause the valves to momentarily reopen. This sequence can be repeated many times until the desired pressure increase in the remote devices is reached.
Similar instability problems can be caused by a pressure difference between the proportional control valves and the remote devices while discharging. The valves can momentarily close off the exhaust of pressurized fluid from the remote devices before the pressure in the remote devices has reduced to a desired level. A static pressure increase in the valves up to the prior average dynamic pressure of the valves and the remote devices can cause the valves to momentarily reopen to continue to exhaust more of the pressurized fluid from the remote devices. Although the desired pressure level is eventually reached while either charging or discharging, the additional openings and closings of the valves, which are required to reach the desired pressure level, can cause accelerated wear, noise, pressure fluctuations, and a decreased response time.
In addition to instability problems caused by restrictions to fluid flow between proportional control valves and remote devices, these valves can also have design problems relating to limiting proportional solenoids to a reasonable size and power rating while sizing the valves to accommodate high rates of fluid flow that are required to achieve fast response times. For example, exhaust passages accommodating high rates of fluid flow are associated with large valve seats over which the back pressure from remote devices is applied. The solenoids are sized to overcome the forces generated by the back pressure applied over the area of the seats to regulate the back pressure proportional to a control force.
My copending application Ser. No. 505,889 reduces solenoid size requirements by providing two separate exhaust passages. One of the passages is sized to accommodate low rates of fluid flow, and the other passage is sized to accommodate high rates of fluid flow. The low-flow passage conveys back pressure to the solenoid, whereas the high-flow passage is opened in response to a pressure increase in the low-flow exhaust. This reduces the size and power requirements of the solenoid but makes the valve more complicated.